Subgingival plaque samples were obtained from 32 cats showing different stages of periodontal disease. Correlations were sought between gingival index scores and the prevalence of various microbial groups, and between microbial populations found in sites designated as most-affected and least-affected within individual cats. The tendency with higher gingival index scores, and with the most-affected sites, was toward a microbial population composed to a greater extent of anaerobic Gram-negative rods. The most common organism was an anaerobic Gram-negative rod in the black-pigmented Bacteroides group that was biochemically similar to B. gingivalis but had catalase activity. The black-pigmented Bacteroides group and Peptostreptococcus anaerobius were found in increasing numbers with increasingly severe periodontal disease. Pasteurella multocida was isolated from most samples and appeared to decrease in numbers with increasing periodontal disease.